mb/acer/g43t-am3: Rework mainboard for variant mechanism

mb/acer/g43t-am3: Rework mainboard for variant mechanism

In preparation for CB:83968, rework the configuration files and move
files specific to G43T-AM3 to its own variant directory.

Change-Id: I425852f4bdacf7cb6688a5fb845ac3001373262e
Signed-off-by: Michael Büchler <michael.buechler@posteo.net>
Signed-off-by: Julia Kittlinger <launchpad.vineyard395@passinbox.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57764
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>